Natural rubber and synthetic butadiene-styrene rubber must be "broken down" or peptized before they are compounded with the various ingredients used in their vulcanization. This has conventionally been accomplished by incorporating a chemical peptizing agent into the rubber by milling on an open rubber mill or by using a Banbury mixer. The effectiveness of the peptization is determined by the decrease in rubber viscosity of the rubber as measured by the Mooney viscosity in accordance with ASTM D1646.
Many peptizing agents have been used in the rubber industry to "break down" rubber in the manner described, including pentachlorothiophenol, zinc pentachlorothiophenate, aryl mercaptans, mercaptobenzothiazole, and the like.
Rubber peptization in the industry is desirably accomplished as rapidly as possible and those peptizing agents or compositions which will reduce the viscosity to the desired level in the shortest period of time at the lowest concentration are preferred. It is also highly desirable to be able to effectively peptize the rubber with a single peptizing agent or composition over a broad range of temperatures e.g., from about 70.degree. C. to as high as about 150.degree. C. Heretofore, such a result has not always been readily accomplished.
For instance, two widely used, commercially accepted products have been available to the rubber industry. One, o,o'-dibenzamidodiphenyl disulfide, has been widely used at high temperatures, i.e., above about 100.degree. C. The other, zinc o-benzamidothiophenate, has been widely used at low temperatures, i.e., up to about 100.degree. C.
The continued acceptance of these two peptizing agents illustrates in general that no single peptizing agent has been available which gives broad temperature peptizing efficacy.